1. Field of the Invention
The present invention relates generally to a coating method, more particularly to a coating method using a slide hopper type coating apparatus with a slide surface in manufacturing a film for photosensitive material, photographic printing paper, and so forth, in which multiple liquid coating compositions (hereunder called xe2x80x9ccoating liquidxe2x80x9d) including gelatin are applied to a web that runs continuously so as to make multiple layers on the web.
2. Description of Related Art
A slide hopper type coating apparatus is one of the apparatuses which coat coating liquid on a running web surface; roughly classifying coatings of such apparatuses, there are slide bead type coating and a slide curtain coating. The slide hopper type coating apparatus has a superior coating capability in a case of simultaneously coating of a plurality of coating liquids on the web at a high speed, and is widely used in manufacturing a film of photosensitive material and photographic printing paper.
In order to attain a coated surface in a satisfactory quality by the use of the slide hopper type coating apparatus, stabilizing a flow of the multiple layered coating liquid flowing down on the slide surface is essential. In a slide bead coating, forming a stable bead is essential, and in a slide curtain coating, eliminating the entrained air effect is essential. Therefore, designing coating liquid composition to satisfy the above conditions is necessary.
Conventionally, a number of measures have been attempted to achieve the above conditions. For example, Japanese Patent Publication No. 4-10053 suggests preventing the entrained air effect in the high-speed coating by lowering the viscosity of the substratum layer (bottom layer) next to the web when being coated. Japanese Patent Publication No. 60-12107 suggests designing composition of the coating liquid so that the viscosity is low when a shearing that is the same as one with the bead is applied in the slide bead coating. Japanese Patent Application Laid-open Nos. 56-108566 and 11-197589 suggest reducing a difference in viscosity between the coating liquid for the bottom layer and the coating liquid for the middle layer in a low shearing by adding thickener, because instability such as ruffles often occurs in the multi-layered coating liquid that flows down on the slide surface when using the coating liquid that is low in viscosity to the bottom layer with respect to the slide surface.
In most of the contents of the above-mentioned prior arts, it has been attempted to improve the coating quality by lowering the viscosity of the coating liquid for the bottom layer in a high shearing while stabilizing the multi-layered coating liquid flowing down on the slide surface by making the viscosity of the coating liquid for the bottom layer similar to the other layers in the low shearing.
However, if the compositions of the coating liquid for the bottom layer and the coating liquid for the middle layer differ, and specially if both of the coating liquids for the bottom layer and the middle layer contain gelatin and the difference in the gelatin concentration of the coating liquids of both layers is large, or, if the maximum inclination angle of the slide surface is at least 40 degrees, that is, the condition in which the flow of the multi-layered coating liquid on the slide surface is often unstable, the instability such as ruffles cannot be prevented by merely applying the conventional technique, and thus the coated surface with a satisfactory quality cannot be obtained.
In such condition, stabilizing the flow of the multi-layered coating liquid on the slide surface is not enough to attain a coated surface with a satisfactory quality; as described above, in the slide bead coating, forming a stable bead is necessary, and in the slide curtain coating, preventing the entrained air effect in the high-speed coating is also necessary.
The present invention has been developed in view of the above-described circumstances, and has as its object the provision of a coating method which can attain a coated surface with a satisfactory quality even in a condition where the flow of the multi-layered coating liquid is often unstable.
In order to achieve the above-described objects, the present invention is directed to a coating method in which a running web is coated with a plurality of coating liquids by a slide hopper type coating apparatus with a slide surface on which the plurality of coating liquids flow down to form a plurality of coating layers on the web, wherein: at least two of the plurality of coating liquids for a bottom layer and a layer next to the bottom layer with respect to the slide surface contain gelatin; a gelatin concentration of the coating liquid for the bottom layer is set to be lower than a gelatin concentration of the coating liquid for the layer next to the bottom layer by at least 2 points in percentage; and the plurality of coating liquids are prepared such that a ratio A/B is not less than 0.05 and not more than 0.2 and a ratio C/D is not less than 0.2 and not more than 0.67, where A is a flow amount per width of the coating liquid for the bottom layer on the slide surface, B is a flow amount per width of the whole of the plurality of coating liquids on the slide surface, C is a viscosity of the coating liquid for the bottom layer, and D is a viscosity of the coating liquid for the layer next to the bottom layer.
The present invention is derived from the knowledge that a coated surface with a satisfactory quality can be attained by setting the viscosity of the coating liquid for the bottom layer in the low shearing to be lower than the coating liquid for the middle layer and at the same time selecting an appropriate flow amount ratio of the coating liquid for the bottom layer with respect to the entire multi-layered coating liquid, even though under the conditions that the flow of the multi-layered coating liquid on the slide surface tends to be unstable where the gelatin concentration of the coating liquid for the bottom layer is low and the slide angle of the slide surface is large. According to the present invention, the gelatin concentration of the coating liquid for the bottom layer is set to be lower than the gelatin concentration of the coating liquid for the middle layer by at least 2 points in percentage, the ratio A/B, which is the flow amount ratio of the coating liquid for the bottom layer with respect to the entire multilayered coating liquid, is set to be between 0.05 and 0.2, and the ratio C/D, which is the viscosity ratio between the coating liquid for the bottom layer and the coating liquid of the middle layer, is set to be between 0.2 and 0.67. Thereby, the flow of the multi-layered coating liquid flowing down on the slide surface can be stabilized.
Preferably, the slide surface has a portion of a maximum inclination angle of not less than 40 degrees with respect to the level. According to the present invention, the multi-layered coating liquid flowing down on the slide surface can have an appropriate flow speed, and moreover the flow of the multi-layered coating liquid on the slide surface can be stabilized.
Preferably, a length of the portion of the slide surface of the maximum inclination angle of not less than 40 degrees is not more than 10 cm. According to the present invention, the flow of the multi-layered coating liquid on the slide surface can be even more easily stabilized while providing the appropriate flow speed to the multi-layered coating liquid flowing down on the slide surface.
Preferably, the gelatin concentration of the coating liquid for the bottom layer is not less than 3% and not more than 8%. According to the present invention, the flow of the multi-layered coating liquid on the slide surface can be even more easily stabilized.